function [ out ] = H(alpha_s,beta_s)

r{1} = [1/4, -1/4, 0];
r{2} = [-1/4, -1/4, 0];
r{3} = [-1/4, 1/4, 0];
r{4} = [1/4, 1/4, 0];

phi = nan(4,1);

[kx,ky,kz] = sph2cart(beta_s, alpha_s, 1);
k = [kx;ky;kz];

for i = 1:4
    phi(i) = 2*pi*r{i}*k;
end

out = exp(1j * phi);
end

